Self-adjusting three-prong electric plug adaptable for either two or three prong electric sockets



March 7, 1967 M. CRAMER ET AL 3,308,415

SELF-ADJUSTING THREE-PRONG ELECTRIC PLUG ADAPTABLE FOR EITHER TWO OR THREE PRONG ELECTRIC SOCKETS Filed Nov. 17, 1964 2 Sheets-Sheet 1 in lllllfllln In [N VENIORS MILTON.L. CRAME'R DOV. Z. GLUCKSMHN mgm M. 1.. CRAMER ET AL 3,308,415 SELF-ADJUSTING THREE-PRONG ELECTRIC PLUG ADAPTABLE FOR EITHER TWO OR THREE PRONG ELECTRIC SOCKETS 2 Sheets-Sheet 2 7 6 l 9 7 l 7 v Q n N C d r a m M m INVENTORS.

MILTON. L. CRAHER DOV.Z. GLUCKSMAN Q ya United States Patent Ofifice 3,308,415 Patented Mar. 7, 1967 3,308,415 SELF-ADJUSTING THREE-PRONG E ECTRIC PLUG ADAPTABLE FOR EITHER TWO R THREE PRONG ELECTRIC SOCKETS Milton L. Cramer, Wolcott St., Litchfield, Conn. 06759, and Dov Z. Glucksman, Torrington, Conn. (26 Chase Ave., West Newton, Mass. 02165) Filed Nov. 17, 1964, Ser. No. 411,748 4 Claims. (Cl. 339-14) The present invention relates to an electrical plug with a convertible feature to allow its insertion into either a two-prong or three-prong electric receptacle.

At present, in order to utilize the standard, non-adjustable three-prong electric plug featuring two rigid blades and a ground pin, an intermediate adapting device must be used. This adaptor receivesthe three-prong plug and engages the receiving two-blade socket with two blades of its own and offers an auxiliary ground wire which may or may not be utilized.-

Another recent design features a three-prong plug, the ground pin being pivoted manually to one side or the other when it cannot be engaged in thereceiving twoblade socket.

The above described design and similar ones in present use represent serious deficiencies from the standpoint of convenience and ease of use. Too much manual adjustment isrequired for the simple and routine operation of using an electrical plug; i

The safety inuse of alljpresent convertible three-prong p lu gs is also violated by thenecessity of manual readjustment in the interchangin 'of use. By merely forgetting 0r neglecting to "readjus'tor reposition the ground pin of the 'plug after the 'plug'has been used in a two-prong socket, the main purpose of the such third prong, i.e., the certainty of grounding, is completely defeated.

The object of the present invention is to avoid the foregoing disadvantages. The present invention represents a self-adjusting plug, the inherent features of which allow it to accomplish its own complete adaptation to the particular situation of use. Both the convenience and safety factors are retained in the use and operation as the following text describes.

The plug body is made of any reasonably solid insulating material. Two fiat blades are either imbedded into or attached rigidly to the plug body, with provision made for the attachment of electrical leads to them.

The distinguishing feature of the present device is its third prong or ground pin which features the capability of linear motion perpendicular to the receiving receotacles face. This pin retracts when the pl g is engaged into a two-blade receptacle, and yet still will protrude and engage its specific receptacle provided in a three-prong soc et.

The ground pin is made of an electrically conductive metal. The pin is designed to engage its receiving portal in a three-prong socket and has a U-shaped cross-section. It is cast, stamped or machined, and the exposed tip of the pin is rounded. Another part of the pin is a cylindrical section, on theperiphery of which helical threads or grooves, as the case may be, are fashioned. The crosssection of the pin is confined within the diameter of the cylinder.

The plug body is provided with a threaded passageway which matches the threads or grooves on the cylindrical part of the pin. The linear motion of the pin is achieved by a relative rotational movement of the pin with regard to the passageway. The end of the passageway from which the pin projects is provided with a collar or similar locking device to prevent ejection of the total pin. The opposite end of the passageway is sealed or closed.

A compressible spring is placed within the passageway between its closed end and the rearface of the cylindrical part of the pin. This spring acts to keep the pin in its utmost projected position at all times, except when retraction of the pin is caused by external force.

Electrical contact between the moving pin and the ground lead terminal is accomplished either by the threads and grooves or by the compressible spring, which, for such purposes, is made of a metal characterized by high electrical conductivity plus great tensile strength, such as beryllium copper.

Since the linear motion of the pin is accompanied by a rotational motion, no linear motion will be experienced when the pin is prevented from rotating. Such will be the case when the U-shaped pin is received into the standard, matching U-shaped socket opening. The mating of these two similar shapes will provide a key effect and thus full introduction of the ground pin and the two rigid blades into the receiving socket can be achieved.

Used in the two-blade socket, the face of the socket will provide resistance to the ground pin. Combined rotational retraction of the ground pin will occur, allowing only the two rigid blades to engage their receiving twoblade socket. When the plug is disengaged from the twoblade socket, the spring-loaded ground pin will reproject.

The present invention will now be discussed with reference to the accompanying drawings in which:

FIGURE 1 is a transverse sectional view of an electrical plug taken along the lines AA of FIGURE 2 and illustrating in particular the ground pin, its threaded passageway and the compressible spring;

FIGURE 2"i's a bottom elevation of the 'same 'electrical plug shown in FIGURE 1, showing the U-shaped ground pin;

FIGURE 3 is a partial section of a modification of the electrical plug taken through a portion of the plug similar to that indicated in FIG. 2; and

FIGURE 4 is a partial section of a further modification of the electrical plug taken through a portion of the plug similar to that indicated in FIG. 2.

Referring now to the drawings and more particularly to FIGURE 1, pin 6 is cast, stamped or machined from an electrically conductive material. It is composed of three functional elements; a prong 6a, a cylinder 6b, and one or more threads 60, which are fashioned on said cylinder 6b. The cross-section of the prong 6a is confined within the diameter of the cylinder 6b.

The sleeve 3 is manufactured of some electrically conductive metal. It is provided with one or more helical grooves 3a along its interior wall exactly matching the pin threads 60. A boss 4 serves as a terminal support for spring 5 and prevents the entrance of foreign matter into the passageway 3b. The opposite end of the sleeve 3 is open and the grooves extend up to the sleeves lip 30 so that the spring 5 and the pin 6 can be introduced into the sleeve. The lip 30 is thereafter bent inwards so as to secure the containment of these components yet allow the free linear motion of the prong 6a.

The plug body 1, made of some reasonably solid insulating material, holds the two fiat blades 2 and the metallic sleeve 3, each of them being connected within the body to their appropriate electrical leads by means of soldering, mechanical attachment or any other convenient manner.

FIGURE 2 is a bottom elevation of the same electrical plug described in FIGURE 1. The U-shaped prong 6a is shown to be confined within the cylindrical section 61) and by the already bent sleeve lip 30. The two blades 2 are shown here in relation to the prong 6a.

FIGURE 3 is a section through the plug body showing in particular the passageway 13, the pin 16, and the spring 14. A helix 12, made of an electrically conductive metal, is embedded into the walls of the passageway so that approximately one-half projects into the passageway 13. A helical groove 16c is fashioned into the cylindrical portion 16b of the pin 16. This groove is mated to the helix present in the passageway 13. A ground lead is attached to the embedded end of the helix in some convenient manner. A constant electrical contact is obtained by the physical contact of the groove 16c and the helix 12. In order to prevent the total ejection of the pin 16 from the passageway 13, a locking device 17, fashi-oned as a washer, is secured at the opening of the passageway 13. The peripheral edge of this washer 17 is threaded and secured into the plug 11 by a matching thread. The washers opening permits the protrusion of the U-shaped prong 16a, yet prevents the ejection of the larger cylinder 16!).

FIGURE 4 is a section through the plug body showing in particular the passageway 23, the pin 26, and the spring 25.

The pin 26 is comprised of a U-shaped prong 26a and a cylindrical section 2617, on which one or more helical threads 260 are fashioned. This thread, or threads 26c, mate with a groove, or grooves 24, that are cut through the walls of the passageway 23 into the plug body 21.

A disc 22, made of an electrically conductive material, is seated at the end of the passageway 23. An electrical lead is attached to this disc in some convenient manner.

A compressible spring 25 is made of a metal characterized by high electrical conductivity plus great tensile strength. This spring is located between the disc 22 and the rear end of the cylinder 26!), so that electric contact is maintained from the electrical lead through the spring to the ground pin.

A locking device 27 is provided which is similar to the washer 17 described in FIGURE 3.

It will be apparent from the foregoing that the present invention provides a three-prong plug which can be used selectively in conjunction with sockets adapted for ground wire connection or not adapted for ground Wire connection without receiving any adjustment of the plug or any rearrangement of any parts thereof.

What is claimed and desired to be secured by United States Letters Patent is:

1. An electrical plug comprising in combination:

(a) a plug body of insulating material defining an internal passageway;

extension inserted into said passageway of said plug body;

l axial cylindrical extension and said ground pin in a direction longitudinally thereof; and

(e) cooperating guide means on said plug body and said axial cylindrical extension for causing a combined rotary and linear motion of said ground pin into said passageway defined by said plug body in" response to axial pressure upon said ground pin opposed to the bias of said spring means. 2. The electrical plug according to claim 1 wherein one 2 or more helical threads are fashioned onsaid axial cylindrical extension matching one or more helical grooves fashioned in the internal periphery of a metallic sleeve contained in said passageway of said plug body.

3. The electrical plug according to claim 1 wherein'a metal helix is partially imbedded in said passageway walls and matching helical grooves that are cut or fashioned on the periphery of said axial cylindrical extension.

4. The electrical plug according to claim 1 wherein helical grooves are cut or cast into said passageway walls 3 and one or more matching helical threads are fashioned on the periphery of said axial cylindrical extension.

References Cited by the Examiner UNITED STATES PATENTS 1,390,607 9/1921 Farmer 339-48 2,323,736 7/1943 Tousley 339-31. 3,219,962 11/1965 Whalen n 33914 FOREIGN PATENTS 40 794,339 12/1935 France.

567,201 12/1932 Germany.

BOBBY R. GAY, Primary Examiner.

P. A. CLIFFORD, Assistant Examiner.

(d) spring means inside said passageway biasing said 

1. AN ELECTRICAL PLUG COMPRISING IN COMBINATION: (A) A PLUG BODY OF INSULATING MATERIAL DEFINING AN INTERNAL PASSAGEWAY; (B) A PAIR OF BLADE CONTACTS EXTENDING GENERALLY PARALLEL TO SAID PASSAGEWAY AND PROJECTING OUTWARDLY FROM A SURFACE OF SAID PLUG BODY; (C) A GROUND PIN NON-CIRCULAR IN CROSS-SECTION EXTENDING GENERALLY PARALLEL TO SAID PAIR OF BLADE CONTACTS AND PROJECTING OUTWARDLY FROM SAID SURFACE OF SAID PLUG BODY, SAID GROUND PIN HAVING AN AXIAL CYLINDRICAL EXTENSION INSERTED INTO SAID PASSAGEWAY OF SAID PLUG BODY; (D) SPRING MEANS INSIDE SAID PASSAGEWAY BIASING SAID AXIAL CYLINDRICAL EXTENSION AND SAID GROUND PIN IN A DIRECTION LONGITUDINALLY THEREOF; AND (E) COOPERATING GUIDE MEANS ON SAID PLUG BODY AND SAID AXIAL CYLINDRICAL EXTENSION FOR CAUSING A COMBINED ROTARY AND LINEAR MOTION OF SAID GROUND PIN INTO SAID PASSAGEWAY DEFINED BY SAID PLUG BODY IN RESPONSE TO AXIAL PRESSURE UPON SAID GROUND PIN OPPOSED TO THE BIAS OF SAID SPRING MEANS. 